Liquid-crystalline medium

ABSTRACT

The invention relates to a liquid-crystalline medium which is characterised in that it comprises one or more compounds of the formula A  
                 
 
     and at least one compound of the formula B  
                 
 
     in which R a , R b , Y, Z 1 , Z 2 , L 1 , L 2 , L 3 , L 4 , L 5 , L 6 , L 7 , L 8  and L 9  are as defined in claim 1.

[0001] The invention relates to a liquid-crystalline-medium, to twisted nematic (TN) and supertwisted nematic (STN) liquid-crystal displays having very short response times and good steepnesses and angle dependencies, and to the novel nematic liquid-crystal mixtures used therein.

[0002] TN displays are known, for example from M. Schadt and W. Helfrich, Appl. Phys. Lett., 18, 127 (1971). STN displays are known, for example from EP 0 131 216 B1; DE 34 23 993 A1; EP 0 098 070 A2; M. Schadt and F. Leenhouts, 17th Freiburg Congress on Liquid Crystals (8.-10.04.87); K. Kawasaki et al., SID 87 Digest 391 (20.6); M. Schadt and F. Leenhouts, SID 87 Digest 372 (20.1); K. Katoh et al., Japanese Journal of Applied Physics, Vol. 26, No.11, L 1784-L 1786 (1987); F. Leenhouts et al., Appl. Phys. Lett. 50 (21),1468 (1987); H. A. van Sprang and H. G. Koopman, J. Appl. Phys. 62 (5), 1734 (1987); T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (10), 1021 (1984), M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (5), 236 (1987) and E. P. Raynes, Mol. Cryst. Liq. Cryst. Letters Vol. 4 (1), pp. 1-8 (1986). The term STN here covers any relatively highly twisted display element having a twist angle with a value of between 160′ and 360′, such as, for example, the display elements according to Waters et al. (C. M. Waters et al., Proc. Soc. Inf. Disp. (New York) (1985) (3rd Intern. Display Conference, Kobe, Japan), STN-LCDs (DE-A 35 03 259), SBE-LCDs (T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021), OMI-LCDs (M. Schadt and F. Leenhouts, Appl. Phys. Lett. 50 (1987), 236, DST-LCDs (EP-A 0 246 842) or BW-STN-LCDs (K. Kawasaki et al., SID 87 Digest 391 (20.6)).

[0003] In particular, STN displays are distinguished compared with standard TN displays by significantly better steepnesses of the electro-optical characteristic line and, associated therewith, better contrast values, and by significantly lower angle dependence of the contrast.

[0004] Of particular interest are TN and STN displays having very short response times, in particular also at relatively low temperatures. In order to achieve short response times, the rotational viscosities of the liquid-crystal mixtures have hitherto been optimised using mostly monotropic additives having relatively high vapour pressure. However, the response times achieved were not adequate for every application.

[0005] In order to achieve a steep electro-optical characteristic line in the displays according to the invention, the liquid-crystal mixtures should have relatively large values for the ratio between the elastic constants K₃ 3/K₁₁ and relatively small values for Δε/ε_(⊥), where Δε is the dielectric anisotropy and ε_(⊥) is the dielectric constant perpendicular to the longitudinal molecular axis.

[0006] In addition to optimisation of the contrast and response times, further important requirements are made of mixtures of this type:

[0007] 1. broad d/p window

[0008] 2. high long-term chemical stability

[0009] 3. high electrical resistance

[0010] 4. low frequency and temperature dependence of the threshold voltage.

[0011] The parameter combinations achieved are still far from adequate, in particular for high-multiplex STN displays (with a multiplex rate in the region of about {fraction (1/400)}), but also for medium- and low-multiplex STN displays (with multiplex rates in the region of about {fraction (1/64)} and {fraction (1/16)} respectively), and TN displays. This is partly attributable to the fact that the various requirements are affected in opposite manners by material parameters.

[0012] A requirement for display applications of relatively low resolution (mux {fraction (1/16)}) is an operating voltage in the region around 3 V at birefringence values of Δn=0.13-0.14. To this end, liquid-crystal mixtures having threshold voltages of less than 1 V and typically in the range V₁₀=0.65-0.75 V must be provided. Liquid-crystal mixtures from the prior art comprising highly polar substances containing a nitrile group, which are already employed up to the solubility limit, generally do not achieve the requisite combination of low threshold voltage and low birefringence.

[0013] Thus, there continues to be a great demand for TN and STN displays, in particular for medium- and low-multiplex STN displays, having very short response times at the same time as a large working-temperature range, high characteristic-line steepness, good angle dependence of the contrast and low threshold voltage which meet the above-mentioned requirements.

[0014] The object of the present invention is to provide mixtures having a low threshold voltage and low birefringence, in particular for TN and STN applications, which comprise compounds which are distinguished by low birefringence and moderate polarity.

[0015] The invention furthermore has the object of providing liquid-crystalline media, in particular for TN and STN displays, which do not have the above-mentioned disadvantages or only do so to a lesser extent and at the same time have short response times, in particular at low temperatures, and very good steepnesses.

[0016] It has now been found that this object can be achieved if use is made of liquid-crystal mixtures which comprise one or more compounds of the formula A

[0017] and at least one compound of the formula B

[0018] in which

[0019] R^(a) and R^(b) are each, independently of one another, H or an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

[0020]  —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another,

[0021] Z¹ and Z² are each, independently of one another, —(CH₂)₄—, —CF₂O—, —COO—, —OCF₂—, —OCH₂—, —CH₂O—, —CH₂—, —(CH₂)₃— or a single bond, in which at least one bridge is —OCF₂— or —CF₂O—,

[0022] L¹ to L⁹ are each, independently of one another, H or F, and

[0023] Y is F, Cl, SF₅, NCS, SCN, CN, OCN, or a monohalogenated or polyhalogenated alkyl, alkoxy, alkenyl or alkenyloxy radical, each having up to 5 carbon atoms.

[0024] The use of substances of the formula A which have moderate polarity and low birefringence gives TN and STN media which are distinguished by low threshold voltages and low Δn values.

[0025] The use of the compounds of the formulae A and B in the mixtures for TN and STN displays according to the invention results in

[0026] high steepness of the electro-optical characteristic line,

[0027] low temperature dependence of the threshold voltage, and

[0028] very fast response times, in particular at low temperatures.

[0029] The compounds of the formulae A and B significantly shorten, in particular, the response times of TN and STN mixtures while simultaneously increasing the steepness and reducing the temperature dependence of the threshold voltage.

[0030] The mixtures according to the invention are furthermore distinguished by the following advantages:

[0031] they have low viscosity,

[0032] they have a low threshold voltage and operating voltage, and

[0033] they effect long shelf lives in the display at low temperatures.

[0034] The invention furthermore relates to a liquid-crystal display having

[0035] two outer plates, which, together with a frame, form a cell,

[0036] a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell,

[0037] electrode layers with alignment layers on the insides of the outer plates,

[0038] a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree to 30 degrees, and

[0039] a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5° and 600°,

[0040] a nematic liquid-crystal mixture consisting of

[0041] a) 15-75% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5;

[0042] b) 25-85% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between −1.5 and +1.5;

[0043] c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below −1.5, and

[0044] d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3,

[0045] characterised in that component A comprises at least one compound of the formula A

[0046] and at least one compound of the formula B

[0047] in which R^(a), R^(b), Z¹, Z², L¹, L², L³, L⁴, L⁵, L⁶, L⁷, L⁸, L⁹ and Y are as defined above.

[0048] The invention also relates to TN and STN displays, in particular medium- and low-multiplexed STN displays, which contain the liquid-crystal mixture according to the invention. The mixtures according to the invention are furthermore used in IPS mixtures.

[0049] Formula A covers, in particular, compounds of the sub-formulae A-1 to A-12

[0050] in which R^(a) is as defined in claim 1.

[0051] Particular preference is given to mixtures according to the invention which comprise at least one compound of the formula A-2 and/or A-6.

[0052] In the formulae A and A-1 to A-12, R^(a) is particularly preferably straight-chain alkyl or alkoxy, 1E-alkenyl or 3E-alkenyl having from 2 to 7 carbon atoms.

[0053] Z¹ is preferably a single bond, and Z² is preferably —CF₂O—. L¹ and L² are preferably F.

[0054] Y is preferably F, Cl, CN, CF₃, C₂F₅, C₃F₇, CF₂H, OCF₃, OCF₂H, OCFHCF₃, OCFHCFH₂, OCFHCF₂H, OCF₂CH₃, OCF₂CFH₂, OCF₂CF₂H, OCF₂CF₂CF₂H, OCF₂CF₂CFH₂, OCFHCF₂CF₃, OCFHCF₂CF₂H, OCFHCFHCF₃, OCH₂CF₂CF₃, OCF₂CF₂CF₃, OCF₂CFHCFH₂, OCF₂CH₂CF₂H, OCFHCF₂CFH₂, OCFHCFHCF₂H, OCFHCH₂CF₃, OCH₂CFHCF₃, OCH₂CF₂CF₂H, OCF₂CFHCH₃, OCF₂CH₂CFH₂, OCFHCF₂CH₃, OCFHCFHCFH₂, OCFHCH₂CF₃, OCH₂CF₂CFH₂, OCH₂CFHCF₂H, OCF₂CH₂CH₃, OCFHCFHCH₃, OCFHCH₂CFH₂, OCH₂CF₂CH₃, OCH₂CFHCFH₂, OCH₂CH₂CF₂H, OCHCH₂CH₃, OCH₂CFHCH₃, OCH₂CH₂CF₂H, OCClFCF₃, OCClFCClF₂, OCClFCFH₂, OCFHCCl₂F, OCClFCF₂H, OCClFCClF₂, OCF₂CClH₂, OCF₂CCl₂H, OCF₂CCl₂F, OCF₂CClFH, OCF₂CClF₂, OCF₂CF₂CClF₂, OCF₂CF₂CCl₂F, OCClFCF₂CF₃, OCClFCF₂CF₂H, OCClFCF₂CClF₂, OCClFCFHCF₃, OCClFCClFCF₃, OCCl₂CF₂CF₃, OCClHCF₂CF₃, OCClFCF₂CF₃, OCClFCClFCF₃, OCF₂CClFCFH₂, OCF₂CF₂CCl₂F, OCF₂CCl₂CF₂H, OCF₂CH₂CClF₂, OCClFCF₂CFH₂, OCFHCF₂CCl₂F, OCClFCFHCF₂H, OCClFCClFCF₂H, OCFHCFHCClF₂, OCClFCH₂CF₃, OCFHCCl₂CF₃, OCCl₂CFHCF₃, OCH₂CClFCF₃, OCCl₂CF₂CF₂H, OCH₂CF₂CClF₂, OCF₂CClFCH₃, OCF₂CFHCCl₂H, OCF₂CCl₂CFH₂, OCF₂CH₂CCl₂F, OCClFCF₂CH₃, OCFHCF₂CCl₂H, OCClFCClFCFH₂, OCFHCFHCCl₂F, OCClFCH₂CF₃, OCFHCCl₂CF₃, OCCl₂CF₂CFH₂, OCH₂CF₂CCl₂F, OCCl₂CFHCF₂H, OCClHCClFCF₂H, OCF₂CClHCClH₂, OCF₂CH₂CCl₂H, OCClFCFHCH₃, OCF₂CClFCCl₂H, OCClFCH₂CFH₂, OCFHCCl₂CFH₂, OCCl₂CF₂CH₃, OCH₂CF₂CClH₂, OCCl₂CFHCFH₂, OCH₂CClFCFCl₂, OCH₂CH₂CF₂H, OCClHCClHCF₂H, OCH₂CCl₂CF₂H, OCClFCH₂CH₃, OCFHCH₂CCl₂H, OCClHCFHCClH₂, OCH₂CFHCCl₂H, OCCl₂CH₂CF₂H, OCH₂CCl₂CF₂H, CH═CF₂, CF═CF₂, OCH═CF₂, OCF═CF₂, CH═CHF, OCH═CHF, CF═CHF or OCF═CHF, in particular F, Cl, CN, CF₃, CF₂H, C₂F₅, C₃F₇, OCF₃, OCF₂H, OCFHCF₃, OCFHCFH₂, OCFHCF₂H, OCF₂CH₃, OCF₂CFH₂, OCF₂CF₂H, OCF₂CF₂CF₂H, OCF₂CF₂CFH₂, OCFHCF₂CF₃, OCFHCF₂CF₂H, OCF₂CF₂CF₃, OCF₂CHFCF₃ or OCClFCF₂CF₃.

[0055] Formula B covers, in particular, compounds of the sub-formulae B-1 to B-6

[0056] in which R^(b) is as defined in claim 1.

[0057] Particular preference is given to compounds in which R^(b) is a straight-chain alkyl radical having 1-7 carbon atoms or an alkenyl radical having 2-7 carbon atoms. Especial preference is given to compounds of the formulae B-1, B-2 and B-4.

[0058] The use of compounds of the formulae A and B in the liquid-crystal mixtures according to the invention results in particularly low rotational viscosity values and in TN and STN displays having high steepness and fast response times, in particular at low temperatures.

[0059] Besides the compounds of the formula A, component A or the liquid-crystalline mixture according to the invention preferably additionally comprises one or more 3,4,5-trifluorophenyl compounds selected from the group consisting of the compounds of the formulae IIa to IIj

[0060] in which

[0061] R² is H or an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

[0062]  , —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another.

[0063] Besides the compounds of the formulae A and B, the medium according to the invention may additionally comprise one or more compounds containing a polar end group, of the formulae II*a to II*s

[0064] in which R² is as defined for R^(a), and L³ and L⁴ are each, independently of one another, H or F. R² in these compounds is particularly preferably alkyl, alkenyl or alkoxy having up to 7 carbon atoms. The medium according to the invention or component A particularly preferably comprises compounds of the formulae IIa, IIb, IIc, IId, IIe, IIf, IIg, IIj, II*b, II*c, II*d, II*f and/or II*i, in particular compounds of the formulae IIa, IIb, IId, IIi, II*a and II*i.

[0065] Besides one or more compounds of the formula B, the mixture according to the invention preferably comprises one or more cyano compounds of the formulae IIIa to IIIi:

[0066] in which R³ is as defined for R^(a), and L¹, L² and L³ are each, independently of one another, H or F. R³ in these compounds is particularly preferably alkyl, alkenyl or alkoxy having up to 7 carbon atoms.

[0067] Particular preference is given to mixtures which comprise one or more compounds of the formulae IIIb, IIIc and IIIf, in particular those in which L¹ and/or L² is F.

[0068] Preference is furthermore given to mixtures which comprise one or more compounds of the formula IIIf and/or IIIg in which L² is H and L¹ is H or F, in particular F.

[0069] The individual compounds of the formulae A, B and of the formulae IIa-IIj, II*a-II*s and IIIa to IIIi and their sub-formulae or also other compounds which can be used in the mixtures or TN and STN displays according to the invention are either known or can be prepared analogously to the known compounds.

[0070] The compounds of the formula A have low viscosities, in particular low rotational viscosities, and low values for the ratio between the elastic constants K₃₃/K₁₁, and therefore result in short response times in the displays according to the invention, while the presence of compounds of the formula B of high dielectric anisotropy, in particular in increased concentrations, causes a reduction in the viscosity.

[0071] Preferred liquid-crystal mixtures comprise one or more compounds of component A, preferably in a proportion of from 15% to 75%, particularly preferably from 20% to 65%. These compounds have a dielectric anisotropy of Δε≧L+3, in particular of Δε≧+8, particularly preferably of Δε≧2+12.

[0072] Further preferred mixtures comprise

[0073] one or more, in particular from two to four, compounds of the formula A,

[0074] in each case one, two or three compounds of the formula A-2,

[0075] one or more, in particular one or two, compounds of the formula B,

[0076] one or more, in particular from two to five, compounds of the formulae IIIa, IIIb, IIIc and/or IIIf.

[0077] Preferred liquid-crystal mixtures comprise one or more compounds of component B, preferably from 25 to 85%. The compounds from group B are distinguished, in particular, by their low values for the rotational viscosity γ₁.

[0078] Component B preferably comprises one or more compounds of the formula IV

[0079] in which

[0080] m is 0 or 1,

[0081] R⁴ is an alkenyl group having from 2 to 7 carbon atoms,

[0082] R⁵ is as defined for R^(a) or, if m=1, is alternatively F, Cl, CF₃ or OCF₃,

[0083] L¹ and L² are each, independently of one another, H or F.

[0084] Particularly preferred compounds of the formula IV are those in which R⁴ is alkenyl having from 2 to 7 carbon atoms, in particular those of the following formulae:

[0085] in which R^(3a) and R^(4a) are each, independently of one another, H, CH₃, C₂H₅ or n-C₃H₇, and alkyl is an alkyl group having from 1 to 7 carbon atoms. Particular preference is given to TN and STN displays according to the invention in which the liquid-crystal mixture comprises at least one compound of the formula IV-1 and/or IV-3 in which R^(3a) and R^(4a) each have the same meaning, and displays in which the liquid-crystal mixture comprises at least one compound of the formula IV-5.

[0086] In a further preferred embodiment, the mixtures according to the invention comprise one or more compounds of the formula IV-6.

[0087] Component B preferably furthermore comprises compounds selected from the group consisting of the bicyclic compounds of the formulae V-1 to V-9

[0088] and/or one or more compounds selected from the group consisting of the tricyclic compounds of the formulae V-10 to V-27

[0089] and/or one or more compounds selected from the group consisting of the tetracyclic compounds of the formulae V-28 to V-34

[0090] in which R⁶ and R⁷ are as defined for R^(a) in claim 1, and L is H or F.

[0091] Particular preference is given to compounds of the formulae V-25 to V-34 in which R⁶ is alkyl and R⁷ is alkyl or alkoxy, in particular alkoxy, each having from 1 to 7 carbon atoms. Preference is furthermore given to compounds of the formulae V-25, V-28 and V-34 in which L is F.

[0092] R⁶ and R⁷ in the compounds of the formulae V-1 to V-34 are particularly preferably straight-chain alkyl or alkoxy having from 1 to 12 carbon atoms.

[0093] Particular preference is given to mixtures according to the invention which comprise one or more compounds of the formula B-3a and/or B-5a

[0094] The mixtures preferably comprise 2-25% by weight, in particular 2-15% by weight, of compounds of the formula B-5a.

[0095] If desired, the liquid-crystalline mixtures comprise an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is greater than 0.2. A multiplicity of chiral dopants, some of which are commercially available, is available to the person skilled in the art for the component, such as, for example, cholesteryl nonanoate, S-811, S-1011 and S-2011 from Merck KGaA, Darmstadt, and CB15 (BDH, Poole, UK). The choice of dopants is not crucial per se.

[0096] The proportion of the compounds of component C is preferably from 0 to 10%, in particular from 0 to 5%, particularly preferably from 0 to 3%.

[0097] The mixtures according to the invention may also, if desired, comprise up to 20% of one or more compounds having a dielectric anisotropy of less than −1,5 (component D).

[0098] If the mixtures comprise compounds of component D, these are preferably one or more compounds containing the structural unit 2,3-difluoro-1,4-phenylene, for example compounds as described in DE-A 38 07 801, 38 07 861, 38 07 863, 38 07 864 or 38 07 908. Particular preference is given to tolans containing this structural unit, as described in International Patent Application PCT/DE 88/00133.

[0099] Further known compounds of component D are, for example, derivatives of 2,3-dicyanohydroquinones or cyclohexane derivatives containing the structural unit

[0100] as described in DE-A 32 31 707 and DE-A34 07 013.

[0101] The liquid-crystal displays according to the invention preferably contain no compounds of component D.

[0102] The term “alkenyl” in the definition of R^(a), R^(b), R², R³, R⁴, R⁵, R⁶ and R⁷ covers straight-chain and branched alkenyl groups, in particular the straight-chain groups. Particularly preferred alkenyl groups are C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl, C₅-C₇-4-alkenyl, C₆-C₇-5-alkenyl and C₇-6-alkenyl, in particular C₂-C₇-1E-alkenyl, C₄-C₇-3E-alkenyl and C₅-C₇-4-alkenyl.

[0103] Examples of preferred alkenyl groups are vinyl, 1E-propenyl, 1E-butenyl, 1E-pentenyl, 1E-hexenyl, 1E-heptenyl, 3-butenyl, 3E-pentenyl, 3E-hexenyl, 3E-heptenyl, 4-pentenyl, 4Z-hexenyl, 4E-hexenyl, 4Z-heptenyl, 5-hexenyl, 6-heptenyl and the like. Groups having up to 5′ carbon atoms are generally preferred.

[0104] Further preferred embodiments relate to liquid-crystal mixtures according to the invention which

[0105] additionally comprise one or more, particularly preferably one, two or three, heterocyclic compounds of the formula Va and/or Vb

[0106] in which

[0107] R⁶ and R⁷ are as defined above, and

[0108] y is F, Cl or OCF₃,

[0109] the proportion of the compounds from the group consisting of Va and Vb is preferably from 2 to 35%, in particular from 5 to 20%,

[0110] additionally comprise one or more, particularly preferably one, two or three, tolan compounds of the formulae T2a, T2b and/or T2c

[0111] in which R⁶ and R⁷ are as defined above.

[0112] The proportion of the compounds from the group consisting of T2a, T2b and/or T2c is preferably from 2 to 20%, in particular from 4 to 12%. The mixture according to the invention preferably comprises two or three compounds of the formula T2a and/or T2b.

[0113] In particularly preferred embodiments, the mixtures comprise

[0114] at least one compound of the formula A-2

[0115] and

[0116] at least one compound of the formula B-1

[0117] at least one compound of the formula IIj;

[0118] at least one tolan compound of the formula T2c;

[0119] at least two, in particular three, compounds of the formula A;

[0120] at least two, in particular three, compounds of the formula B;

[0121] at least one compound of the formula T2a and at least one compound of the formula T2b;

[0122] at least one compound of the formula VI

[0123] in which L¹ is H or F;

[0124] at least one compound of the formula VII

[0125] in which alkyl and alkyl* are each, independently of one another, an alkyl group having from 1 to 7 carbon atoms;

[0126] at least from 2 to 30% by weight, preferably 2-25% by weight, in particular from 3 to 15% by weight, of compounds of the formula VII;

[0127] at least 2.5% by weight of the compounds of the formula IV-5;

[0128] 5-30% by weight, preferably 10-25% by weight, of the compounds of the formula A;

[0129] 25-5-30% by weight, preferably 5-20% by weight, of the compounds of the formula B;

[0130] at least three homologues of the compounds of the formula A, where Ra is then preferably C₂H₅, n-C₃H₇ or n-C₅H₁₁.

[0131] Further particularly preferred embodiments relate to liquid-crystal mixtures which comprise

[0132] a total of from three to six compounds of the formulae A and B, where the proportion of these compounds with respect to the mixture as a whole is from 25 to 65%, in particular from 30 to 55%,

[0133] more than 20% of compounds of positive dielectric anisotropy, in particular having Δε≧+12.

[0134] The mixtures according to the invention are distinguished, in particular on use in TN and STN displays of high layer thicknesses, by very low total response times (t_(tot)=t_(on)+t_(off)).

[0135] The liquid-crystal mixtures used in the TN and STN cells according to the invention are dielectrically positive, with Δε≧1. Particular preference is given to liquid-crystal mixtures with Δε≧3, in particular with Δε≧5.

[0136] The liquid-crystal mixtures according to the invention have favourable values for the threshold voltage V_(10/0/20) and for the rotational viscosity γ₁. If the value for the optical path difference d·Δn is pre-specified, the value for the layer thickness d is determined by the optical anisotropy Δn. In particular at relatively high values for d·Δn, the use of liquid-crystal mixtures according to the invention having a relatively high value for the optical anisotropy is generally preferred, since the value for d can then be selected to be relatively small, which results in more favourable values for the response times. However, liquid-crystal displays according to the invention which contain liquid-crystal mixtures according to the invention with smaller values for Δn are also characterised by advantageous values for the response times.

[0137] The liquid-crystal mixtures according to the invention are furthermore characterised by advantageous values for the steepness of the electro-optical characteristic line, and can be operated with high multiplex rates, in particular at temperatures above 20° C. In addition, the liquid-crystal mixtures according to the invention have high stability and favourable values for the electrical resistance and the frequency dependence of the threshold voltage. The liquid-crystal displays according to the invention have a large working-temperature range and good angle dependence of the contrast.

[0138] The construction of the liquid-crystal display elements according to the invention from polarisers, electrode base plates and electrodes having a surface treatment such that the preferential alignment (director) of the liquid-crystal molecules in each case adjacent thereto is usually twisted by a value of from 160° to 720° from one electrode to the other corresponds to the usual structure for display elements of this type. The term “usual structure” here is broadly drawn and also covers all derivatives and modifications of the TN and STN cell, in particular also matrix display elements and display elements containing additional magnets.

[0139] The surface tilt angle at the two outer plates may be identical or different. Identical tilt angles are preferred. Preferred TN displays have pre-tilt angles between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0° to 7°, preferably from 0.01° to 50, in particular from 0.1 to 2°. In the STN displays, the pre-tilt angle is from 1° to 30°, preferably from 10 to 12° and in particular from 3° to 10°.

[0140] The twist angle of the TN mixture in the cell has a value of between 22.50 and 170°, preferably between 45° and 130° and in particular between 800 and 115°. The twist angle of the STN mixture in the cell from alignment layer to alignment layer has a value of between 100° and 600°, preferably between 170° and 300° and in particular between 180° and 270°.

[0141] The liquid-crystal mixtures which can be used in accordance with the invention are prepared in a manner which is conventional per se. In general, the desired amount of the components used in lesser amount is dissolved in the components making up the principal constituent, advantageously at elevated temperature. It is also possible to mix solutions of the components in an organic solvent, for example in acetone, chloroform or methanol, and to remove the solvent again, for example by distillation, after thorough mixing.

[0142] The dielectrics may also comprise further additives which are known to the person skilled in the art and are described in the literature. For example, 0-15% of pleochroic dyes, stabilisers, antioxidants, UV absorbers, etc., may be added. A particularly suitable UV absorber is a benzotriazole derivative, such as, for example, Tinuvin® from Ciba-Geigy.

[0143] In the present application and in the examples below, the structures of the liquid-crystal compounds are indicated by means of acronyms, the transformation into chemical formulae taking place in accordance with Tables A and B below. All radicals C_(n)H_(2n+1) and C_(m)H_(2m+1) are straight-chain alkyl radicals having n and m carbon atoms respectively. The alkenyl radicals have the trans-configuration. The coding in Table B is self-evident. In Table A, only the acronym for the parent structure is indicated. In individual cases, the acronym for the parent structure is followed, separated by a dash, by the code indicated in the table below for the substituents R^(1*), R^(2*), L^(1*), L^(2*) and L^(3*). Code for R^(1*), R^(2*), L^(1*), L^(2*), L^(3*) R^(1*) R^(2*) L^(1*) L^(2*) L^(3*) nm C_(n)H_(2n+1) C_(m)H_(2m+1) H H H nOm C_(n)H_(2n+1) OC_(m)H_(2m+1) H H H nO.m OC_(n)H_(2n+1) C_(m)H_(2m+1) H H H n C_(n)H_(2n+1) CN H H H nN.F C_(n)H_(2n+1) CN H H F nN.F.F C_(n)H_(2n+1) CN H F F nF C_(n)H_(2n+1) F H H H nOF OC_(n)H_(2n+1) F H H H nF.F C_(n)H_(2n+1) F H H F nmF C_(n)H_(2n+1) C_(m)H_(2m+1) F H H nOCF₃ C_(n)H_(2n+1) OCF3 H H H n-Vm C_(n)H_(2n+1) —CH═CH—C_(m)H_(2m+1) H H H nV-Vm C_(n)H_(2n+1)—CH═CH— —CH═CH—C_(m)H_(2m+1) H H H

[0144] The TN and STN displays preferably contain liquid-crystalline mixtures composed of one or more compounds from Tables A and B. TABLE A (L¹*, L²*, L³* = H or F)

[0145] TABLE B

[0146] TABLE C Table C shows dopants which are usually employed in the mixtures according to the invention.

[0147] TABLE D Stabilisers which can be added, for example, to the mixtures according to the invention are shown below.

[0148] The following examples are intended to illustrate the invention without representing a limitation. The following abbreviations are used:

[0149] cl.p. clearing point (nematic-isotropic phase transition temperature)

[0150] S-N smectic-nematic phase transition temperature

[0151] ν₂₀ flow viscosity (mm²/s, unless stated otherwise, at 20° C.)

[0152] Δ_(n) optical anisotropy (589 nm, 20° C.)

[0153] Δε dielectric anisotropy (1 kHz, 20° C.)

[0154] γ₁ rotational viscosity (mPa·s at 20° C.) steep characteristic-line steepness=V_(go)/V₁₀

[0155] V₁₀ threshold voltage=characteristic voltage at a relative contrast of 10%

[0156] V₉₀ characteristic voltage at a relative contrast of 90%

[0157] t_(ave) $\frac{t_{on} + t_{off}}{2}$

[0158]  (average response time)

[0159] t_(on) time from switching on until 90% of the maximum contrast is reached

[0160] t_(off) time from switching off until 10% of the maximum contrast is reached

[0161] mux multiplex rate

[0162] t_(store) low-temperature storage stability in hours (−20° C., −30° C., −40° C.)

[0163] Above and below, all temperatures are given in ° C. The percentages are percent by weight. All values relate to 20° C., unless stated otherwise. The displays are addressed, unless stated otherwise, without multiplexing.

EXAMPLE 1

[0164] ME2N.F 11.00% Clearing point [° C.]: 90.0 ME3N.F 11.00% Δn [589.3 nm, 20° C.]: 0.132 ME4N.F 14.00% d · Δn [20° C., μm]: 0.85 ME5N.F 15.00% Twist [°]: 240 CCQU-2-F 5.00% V₁₀ [V]: 0.95 CCQU-3-F 9.00% V₉₀/V₁₀: 1.10 CCQU-5-F 4.00% CCG-V-F 10.00% CCP-V-1 7.00% CCPC-33 4.00% CCPC-34 5.00% CCPC-35 5.00%

EXAMPLE 2

[0165] PCH-3N.F.F 12.00% Clearing point [° C.]: 91.5 ME2N.F 8.00% Δn [589.3 nm, 20° C.]: 0.129 ME3N.F 8.00% d · Δn [20 ° C., μm]: 0.85 ME4N.F 11.00% Twist [°]: 240 ME5N.F 11.00% V₁₀ [V]: 0.89 HP-3N.F 4.00% V₉₀/V₁₀: 1.10 HP-4N.F 4.00% HP-5N.F 4.00% CCQU-2-F 5.00% CCQU-3-F 10.00% CCQU-5-F 5.00% CCG-V-F 3.00% CCPC-33 5.00% CCPC-34 5.00% CCPC-35 5.00%

EXAMPLE 3

[0166] PCH-3N.F.F 7.00% Clearing point [° C.]: 84.0 ME2N.F 10.00% Δn [589.3 nm, 20° C.]: 0.141 ME3N.F 10.00% d · Δn [20 ° C., μm]: 0.85 ME4N.F 14.00% Twist [°]: 240 ME5N.F 14.00% V₁₀ [V]: 0.78 HP-3N.F 6.00% V₉₀/V₁₀: 1.13 HP-4N.F 5.00% HP-5N.F 5.00% CCQU-2-F 5.00% CCQU-3-F 9.00% CCQU-5-F 5.00% CCPC-33 5.00% CCPC-34 5.00%

EXAMPLE 4

[0167] PCH-3N.F.F 7.00% Clearing point [° C.]: 62.5 ME2N.F 10.00% Δn [589.3 nm, 20° C.]: 0.141 ME3N.F 10.00% d · Δn [20 ° C., μm]: 0.85 ME4N.F 14.00% Twist [°]: 240 ME5N.F 14.00% V₁₀ [V]: 0.68 HP-3N.F 6.00% V₉₀/V₁₀: 1.15 HP-4N.F 6.00% HP-5N.F 6.00% CCQU-2-F 5.00% CCQU-3-F 6.00% CCQU-5-F 5.00% CCPC-33 4.00% PZU-V2-N 7.00% 

1. Liquid-crystalline medium, characterised in that it comprises one or more compounds of the formula A

and at least one compound of the formula B

in which R^(a) and R^(b) are each, independently of one another, H or an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

—CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another, Z¹ and Z² are each, independently of one another, —(CH₂)₄—, —CF₂O—, —COO—, —OCF₂—, —OCH₂—, —CH₂O—, —CH₂—, —(CH₂)₃— or a single bond, in which at least one bridge is —OCF₂— or —CF₂O—, L¹ to L⁹ are each, independently of one another, H or F, and Y is F, Cl, SF₅, NCS, OCN, CN, SCN, or a monohalogenated or polyhalogenated alkyl, alkoxy, alkenyl or alkenyloxy radical, each having up to 5 carbon atoms.
 2. Liquid-crystalline medium according to claim 1, characterised in that it comprises at least one compound of the formulae A-1 to A-12

in which R^(a) is as defined in claim
 1. 3. Liquid-crystalline medium according to claim 1 or 2, characterised in that it comprises at least one compound of the formulae B-1 to 8-6

in which R^(b) is as defined in claim
 1. 4. Liquid-crystalline medium according to one of claims 1 to 3, characterised in that it additionally comprises at least one compound of the formulae IIa to IIj

in which R² is an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another.
 5. Liquid-crystalline medium according to one of claims 1 to 4, characterised in that it additionally comprises one or more cyano compounds of the formulae IIIa to IIIi

worin R³ is an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another, and L¹, L² and L³ are each, independently of one another, H or F.
 6. Liquid-crystalline medium according to one of claims 1 to 5, characterised in that it additionally comprises one or more compounds of the formula IV

in which m is 0 or 1, R⁴ is an alkenyl group having from 2 to 7 carbon atoms, R⁵ is as defined for R^(a) or, if m 1, is alternatively F, Cl, CF₃ or OCF₃, L¹ and L² are each, independently of one another, H or F.
 7. Liquid-crystalline medium according to one of claims 1 to 6, characterised in that the medium additionally comprises one or more compounds of the formula VII

in which alkyl and alkyl* are each, independently of one another, an alkyl group having from 1 to 7 carbon atoms.
 8. Liquid-crystalline medium according to one of claims 1 to 7, characterised in that the medium additionally comprises one or more tolan compounds of the formulae T2a, T2b and/or T2c

in which R⁶ and R⁷ are an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another.
 9. Liquid-crystalline medium according to one of claims 1 to 8, characterised in that the medium comprises 5-30% by weight of compounds of the formula A.
 10. Liquid-crystalline medium according to one of claims 1 to 9, characterised in that the medium comprises 5-30% by weight of compounds of the formula B.
 11. Liquid-crystalline medium according to one of claims 1 to 10, characterised in that it comprises more than 20% of compounds having a dielectric anisotropy of Δε≧+12.
 12. Use of the liquid-crystalline medium according to claim 1 for electro-optical purposes.
 13. Electro-optical liquid-crystal display containing a liquid-crystalline medium according to claim
 1. 14. TN or STN liquid-crystal display having two outer plates, which, together with a frame, form a cell, a nematic liquid-crystal mixture of positive dielectric anisotropy located in the cell, electrode layers with alignment layers on the insides of the outer plates, a tilt angle between the longitudinal axis of the molecules at the surface of the outer plates and the outer plates of from 0 degree to 30 degrees, and a twist angle of the liquid-crystal mixture in the cell from alignment layer to alignment layer with a value of between 22.5° and 600°, a nematic liquid-crystal mixture consisting of a) 15-75% by weight of a liquid-crystalline component A consisting of one or more compounds having a dielectric anisotropy of greater than +1.5; b) 25-85% by weight of a liquid-crystalline component B consisting of one or more compounds having a dielectric anisotropy of between −1.5 and +1.5; c) 0-20% by weight of a liquid-crystalline component D consisting of one or more compounds having a dielectric anisotropy of below −1.5, and d) if desired, an optically active component C in such an amount that the ratio between the layer thickness (separation of the outer plates) and the natural pitch of the chiral nematic liquid-crystal mixture is from about 0.2 to 1.3, characterised in that component A comprises at least one compound of the formula A

and at least one compound of the formula B

in which R^(a) and R^(b) are each, independently of one another, H or an alkyl radical having from 1 to 12 carbon atoms which is unsubstituted, monosubstituted by CN or CF₃ or at least monosubstituted by halogen, where one or more CH₂ groups in these radicals may also each, independently of one another, be replaced by —O—, —S—,

 —CH═CH—, —C≡C—, —CO—, —CO—O—, —O—CO— or —O—CO—O— in such a way that O atoms are not linked directly to one another, Z¹ and Z² are each, independently of one another, —(CH₂)₄—, —CF₂O—, —OCF₂—, —OCH₂—, —CH₂O—, —CH₂—, —(CH₂)₃— or a single bond, in which at least one bridge is —OCF₂— or —CF₂O—, L¹ to L⁹ are each, independently of one another, H or F, and Y is F, Cl, SF₅, NCS, OCN, CN, SCN, or a monohalogenated or polyhalogenated alkyl, alkoxy, alkenyl or alkenyloxy radical, each having from 1 to 5 carbon atoms. 